Carbon is known to have four unique crystalline structures, including diamond, graphite, fullerene, and carbon nano-tubes (and its larger-diameter cousins—carbon nano-fibers or CNFs). The carbon nano-tube (CNT) refers to a tubular structure grown with a single wall or multi-wall, which can be conceptually obtained by rolling up a graphene sheet or several graphene sheets to form a concentric hollow structure. A graphene sheet or basal plane is composed of carbon atoms occupying a two-dimensional hexagonal lattice. Carbon nano-tubes have a diameter on the order of a few nanometers to a hundred nanometers. Carbon nano-tubes can function as either a conductor or a semiconductor, depending on the rolled shape and the diameter of the tubes. Its longitudinal, hollow structure imparts unique mechanical, electrical and chemical properties to the material. Carbon nano-tubes are believed to have great potential for use in field emission devices, hydrogen fuel storage, rechargeable battery electrodes, and as composite reinforcements.
However, CNTs are extremely expensive due to the low yield and low production rates commonly associated with all of the current CNT preparation processes, such as arc discharge, laser ablation, chemical vapor deposition (CVD), and catalytic CVD (CCVD). The high material costs have significantly hindered the widespread application of CNTs. Earlier CNT production methods include those disclosed in the following patents: H. G. Tennent, “Carbon Fibrils, Method for Producing Same and Compositions Containing Same,” U.S. Pat. No. 4,663,230 (May 5, 1987); C. Snyder, “Carbon Fibrils,” U.S. Pat. No. 5,707,916 (Jan. 13, 1998).
Carbon nano-fibers (CNFs) are prepared from CVD, CCVD, or electro-spinning of polymer nano-fibers followed by carbonization. Electro-spinning has not been regarded as a mass-production method due to the limited amount of material that can be electro-spun with one hollow needle head. An example of the process to produce polymer nano-fibers via electro-spinning is given in D. H. Reneker, et al, “Processes for Producing Fibers and Their Use,” US Pub. No. 2009/0039565 (Feb. 12, 2009).
The CNFs produced by the CVD and CCVD processes are commonly referred to as vapor-grown carbon nano-fibers (VG-CNFs). VG-CNFs have been extensively investigated in recent years and are commercially available at very high prices (e.g., $300/Kg). The following are some examples of CNF production processes: D. J. C. Yates, et al., “Production of Carbon Filaments,” U.S. Pat. No. 4,565,683 (Jan. 21, 1986); S. H. Yoon, “Ultra-fine Fibrous Carbon and Preparation Method Thereof,” US Pub. No. 2009/0075077 (Mar. 19, 2009); S. H. Yoon, “Ultra-fine Fibrous Carbon and Preparation Method Thereof,” U.S. Pat. No. 7,470,418 (Dec. 30, 2008); S. H. Yoon, “Porous Filamentous Nano Carbon and Method of Forming the Same,” US Pub. No. 2009/0004095 (Jan. 1, 2009); G. Oriji, “Carbon Nano Fiber, Production and Use,” US Pub. No. 2009/0008611 (Jan. 8, 2009); J. L. Gonzales Moral, et al., “Carbon Nanofibers and Procedure for Obtaining Said Nanofibers,” US Pub. No. 2009/0035569 (Feb. 5, 2009).
VG-CNFs and related CNTs have several drawbacks that have significantly constrained their scope of application:                (a) Both CVD and CCVD processes typically involve using a catalyst and the catalyst particles (e.g., transition metal nano particles or their alloys) usually become part of the resulting CNF or CNT structure. Normally, there is a significant amount of catalyst used in these processes. The residual catalyst, even just a trace amount, is considered undesirable in many applications. For instance, Fe is viewed as detrimental to the performance of a lithium ion battery if CNFs or CNTs are used as an anode active material. Catalytic particles can also catalyze or accelerate thermal or chemical degradation of a polymer matrix composite material.        (b) The CVD or CCVD processes intrinsically introduce a significant amount of impurities into the resulting CNFs or CNTs. It is not unusual to find a purity level (graphitic carbon content) in a CNF less than 80-90%.        (c) Depending upon the processing conditions, the graphene planes in different CNFs may be oriented at different angles with respect to the fiber axis. Furthermore, the graphene planes may be curved as a cup-shape or a cone-helix structure, which are not conducive to achieving high strength or modulus along the fiber axis. In one example, the fibers consist primarily of conical nano-fibers, but can contain a significant amount of bamboo nano-fibers. Most conical nano-fibers consist of an ordered inner layer and a disordered outer layer. When subjected to a thermal treatment above 1,500° C., some CNFs can undergo a structural transformation with the ordered inner layers changing from a cone-helix structure to a highly ordered multiwall stacked cone structure. The bamboo nano-fibers can have a tapered multiwall nanotube structure for the wall and a multi-shell fullerene structure for the cap of each segment, surrounded by a disordered outer layer. When these fibers are heat treated, the disordered outer layers transform to an ordered multiwall nanotube structure and merge with the wall of each segment. The end caps of each segment transform from a smooth multiwall fullerene structure to one consisting of disjointed graphene planes. Such a thermally induced instability in the CNF structure is an undesirable feature of CNFs for high-temperature applications (e.g., as a reinforcement in a carbon matrix composite).        (d) The CNFs typically have a continuous thermal carbon overcoat, which is a result of the thermal decomposition effect during the CNF formation process via the CVD, CCVD, or carbonization of electro-spun polymer nano-fibrils. Although this carbon overcoat could serve as a protective layer for the internal graphitic crystallites in some applications, the overcoat is detrimental to many other engineering applications. For instance, this overcoat makes it difficult to chemically functionalize the CNF surface, thereby inhibiting the formation of a strong bond between a CNF and a polymer matrix in a polymer composite. In a similar manner, a CNT has a complete, continuous graphene plane wrapped around the tube axis, which has few active sites where chemical functionalization can occur. Hence, chemical functionalization occurs only at the edge unless this surface is chemically treated (e.g., with a strong oxidizing agent, such as fuming sulfuric acid and nitric acid).        (e) In most of the VG-CNFs, the graphene planes or graphitic crystallites are oriented at a non-zero angle with respect to the fiber axis, resulting in a lower strength, modulus, thermal conductivity, and electrical conductivity along the fiber axis direction as compared with fibrils having all graphene planes substantially parallel to the fiber axis (e.g., CNTs).        
Hence, it is desirable to have a carbon or graphite sub-micron fiber (herein referred to as submicron graphitic fibril, SGF) that has a well-controlled, consistent, and stable structure to ensure consistent properties and performance.
It is further desirable to have a low-cost process that is capable of producing sub-micron graphitic fibrils (SGFs) in large quantities.
It is also desirable to have submicron graphitic fibrils that are pure and catalyst-free. It is still further desirable to have SGFs that exhibit much more surface areas for interactions with a chemical species or a matrix material.
It is most desirable to have a SGF that exhibits a higher strength, modulus, thermal conductivity, and electrical conductivity as compared with conventional vapor grown CNFs.
The main object of the present invention is to provide chemically functionalized submicron-scaled graphitic fibrils that exhibit these desirable attributes. A primary goal of producing chemically functionalized SGFs is improved solubility of SGFs in a liquid medium, improved dispersibility of SGFs in a matrix material, or enhanced interfacial bonding between SGFs and a matrix material in a composite.
The present invention provides chemically functionalized submicron graphitic fibrils that are produced by a process that includes (a) exfoliating or splitting existing micron-scaled carbon fibers or graphite fibers (typically >6 μm in diameter and more typically in the vicinity of 12 μm) that are produced from various fiber precursors, e.g. polyacrylonitrile, pitch, cellulose, and rayon) and (b) separating and isolating individual fibrils from the resultant split or exfoliated fibers.
In a related art, an exfoliated graphite fiber was disclosed by D. D. L. Chung, in “Exfoliated Graphite Fibers and Associated Method,” U.S. Pat. No. 4,915,925 (Apr. 10, 1990). However, according to Chung, “Graphite fibers are exfoliated to produce a fiber of reduced density, increased diameter, and flexibility with respect to graphite fibers prior to exfoliation” (see Abstract of U.S. Pat. No. 4,915,925). Chung did not expressly disclose or implicitly suggest the production of submicron or nano-scaled graphitic fibrils from the exfoliated graphite fibers via mechanical shearing or cutting. Chung did not recognize or realize the significance of isolated graphitic fibrils. As a matter of fact, the objective of Chung's process of exfoliation was to reduce the density and increase the diameter of the graphite fibers through exfoliation, which was completely opposite to the objectives of the present invention. Our main objective was to extract submicron fibrils from the internal structure of a carbon or graphite fiber and these submicron graphitic fibrils have higher density and much lower diameters than the original carbon or graphite fiber. Exfoliation was merely a convenient means of opening up the internal structure of an existing micron-scaled carbon fiber to facilitate the subsequent separation or isolation treatment.
Exfoliation of carbon fibers was also studied by M. Toyoda, et al.: “Exfoliation of Carbon Fibers through Intercalation Compounds Synthesized Electrochemically,” Carbon, 39 (2001) 1697-1707; “Intercalation of Nitric Acid into Carbon Fibers,” Carbon, 39 (2001) 2231-2237; “Intercalation of Formic Acid into Carbon Fibers and their Exfoliation,” Synthetic Metals, 130 (2002) 39-43; “Exfoliation of Nitric Acid Intercalated Carbon Fibers,” Carbon, 41 (2003) 731-738; “Exfoliation of Carbon Fibers,” Journal of Physics and Chemistry of Solids, 65 (2004) 109-117; “Preparation of Intercalation Compounds of Carbon Fibers through Electrolysis Using Phosphoric Acid Electrolyte and their Exfoliation,” Journal of Physics and Chemistry of Solids, 67 (2006) 1178-1181; “Study of Novel Carbon Fiber Composite Used Exfoliated Carbon Fibers,” Materials Science and Engineering, B 161 (2009) 202-204.
Again, just like Chung, Toyoda et al did not expressly disclose or implicitly suggest the production of sub-micron graphitic fibrils from exfoliated graphite fibers via mechanical shearing or cutting of the interconnections between the constituent fibrils of an exfoliated carbon fiber. Toyoda et al did not recognize or realize the significance of isolated graphitic fibrils. Again, the isolation or separation of the interconnected constituent fibrils in an exfoliated or split carbon fiber is a critical step in the production of the presently invented graphitic fibrils. This critical step was not taught in either Chung or Toyoda, et al.
Further, Toyoda et al exfoliated carbon fibers as a means of accelerating the graphitization of carbon fibers, as disclosed in “Acceleration of Graphitization in Carbon Fibers through Exfoliation,” Carbon, 42 (2004) 2567-2572. Similarly, Zhang et al split carbon fibers to facilitate the graphitization procedure, as disclosed in F. Zhang, et al. “Effect of Fiber Splitting on the Catalytic Graphitization of Electroless Ni—B-Coated Polyacrylonitrile-Based Carbon Fibers,” Surface & Coating Technology, 203 (2008) 99-103. Neither case was directed at creating isolated/separated graphitic fibrils.
Furthermore, Chung, Toyoda, et al, and Zhang, et al, individually or in combination, failed to teach about chemically functionalizing the exfoliated fibers or functionalizing the separated fibrils isolated therefrom. No prior art taught about how chemical functionalization of graphitic fibrils would affect the properties of the resulting products comprising functionalized graphitic fibrils.
It may be further noted that the instant applicants have previously disclosed a nano graphene platelet (NGP) having a thickness no greater than 100 nm and a length-to-width ratio no less than 3 (preferably greater than 10) from a carbon or graphite fiber using a seemingly similar but actually distinct process [Zhamu et al., “Nano-scaled graphene platelets with a high length-to-width aspect ratio,” US Pub. No. 2009/0155578 (Jun. 18, 2009)]. The NGP with a high length-to-width ratio was prepared by using a method comprising (a) intercalating a carbon fiber or graphite fiber with an intercalate to form an intercalated fiber; (b) exfoliating the intercalated fiber to obtain an exfoliated fiber comprising graphene sheets or flakes; and (c) separating the graphene sheets or flakes to obtain nano-scaled graphene platelets (NGPs). Step (a) in our earlier disclosure was typically carried out to the extent that the intercalating agent significantly penetrates the bulk of the graphite crystallites so that the subsequent exfoliation step produced ultra-thin nano graphene sheets or NGPs that are typically thinner than 100 nm, but more typically thinner than 1 nm (as indicated in claim 6 of US 2009/0155578). In contrast, the intercalation/oxidation step disclosed in the instant application is carried out to the extent that the exfoliated fiber has their constituent fibrils having a diameter or thickness greater than 100 nm but less than 1 μm. This latter submicron graphitic fibril is not a nano material and exhibits dramatically different properties than the nano graphene platelet disclosed earlier.